The present invention relates to a transverse-mode-coupled surface acoustic wave resonator filter, referred to in the following as a transverse-mode resonator filter, with reflectors and input and output converters with normal finger structure arranged between the reflectors.
Resonator filters of this type are known, for example, from xe2x80x9c1992, Ultrasonics Symposiumxe2x80x9d, pages 39-43. The basic construction of the filter is illustrated schematically in FIG. 5. The resonator filter contains two resonators RES. Each of the resonators has an interdigital converter W9 and W10, respectively, with a uniform normal finger structure. Two short-circuited reflectors R1, R1 and R3, R3, respectively, are arranged in each case on opposite sides of the interdigital converters W9 and W10. The reflectors are electrically and acoustically coupled to one another via a common busbar 2, which acts as a ground rail. The busbar 2 is electrically accessible via ground terminals.
In the filter shown in FIG. 5, the interdigital converter W9 is assigned to the filter input, and the interdigital converter W10 to the filter output. All the transverse-mode resonator filters produced so farxe2x80x94that is to say this known filter as wellxe2x80x94have identical input and output converters, and thus identical input and output impedances.
In order to match a surface acoustic wave filter to its circuit environment with as few problems as possible, however, the real part of the filter input impedance and of the filter output impedance must in each case be identical to the impedance of the stage which precedes the filter input and follows the filter output, respectively. This requirement is satisfied only in very rare cases, for which reason complex circuit measures are often required.
It is accordingly an object of the invention to provide a transverse mode resonator filter, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type, that is to say to provide a way by means of which it is possible to create filters with input and output impedances which differ and are matched appropriately.
With the foregoing and other objects in view there is provided, in accordance with the invention, a transverse-mode resonator filter, comprising:
an input converter with a normal finger structure;
an output converter with a normal finger structure acoustically coupled to the input converter and having a given active finger overlap with the finger structure of the input converter;
reflectors disposed adjacent the input and output converters;
one of the input and output converters having an increased impedance due to a reduced active finger overlap with respect to a normal finger structure defined by identical input and output converters.
In accordance with the invention, the reduction of the active finger overlap may be effected by any of the following three measures: The converter length of the input converter or the output converters is shortened relative to the other converter; the active finger overlap of the input converter or the output converter is reduced by omission weighting; or the active finger overlap of the input converter or the output converter is reduced by overlap weighting.
In accordance with an added feature of the invention, a free space formed by the shortened converter length is filled with additional reflectors.
In accordance with an additional feature of the invention, the omission weighting is partially provided by blind fingers.
In further summary, the object of the invention is satisfied with the transverse-mode resonator filter having input and output converters with a normal finger structure, in which the impedance of the input and/or output converters is increased by reducing the original active finger overlap of the normal finger structure and, if the finger overlap of both converters is reduced, for the amount of the reduction to be different.
It is possible to connect several of the above-outlined transverse-mode resonator filters in series and/or in parallel.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a transverse-mode resonator filter, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.